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PaO2 FiO2 Interpretation in ACS — FRCA Final MCQ

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HardIntensive Care MedicinePaO2 FiO2 Interpretation in ACSFRCA Final

A 45-year-old man (ASA II) is in the ICU with acute pancreatitis complicated by abdominal compartment syndrome. His bladder pressure is 32 mmHg. He is on FiO2 0.5, PEEP 12 cmH2O. His PaO2/FiO2 ratio appears to show moderate ARDS. However, which important caveat applies to PaO2/FiO2 interpretation in abdominal compartment syndrome?

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Correct answer: C — The elevated intra-abdominal pressure reduces chest wall compliance, mimicking ARDS on PaO2/FiO2 calculation

Abdominal compartment syndrome causes extrinsic compression of the thorax via diaphragmatic splinting, reducing chest wall compliance. This causes atelectasis, reduced FRC, and impaired oxygenation that mimics ARDS on PaO2/FiO2 ratios. However, the lung parenchyma itself may be normal – the oxygenation impairment is due to extrapulmonary restriction. This is clinically important because: (1) decompressive laparotomy may dramatically improve oxygenation, (2) the ARDS ventilation strategy may need modification (higher PEEP to counteract abdominal pressure), and (3) transpulmonary pressure monitoring may better guide ventilation than airway pressures alone.

Reference: WSACS – 2013 – ACS consensus definitions; ICS – 2023 – ACS management